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I'm thinking of setting up a cloud backup for home computers, and to increase privacy I want to encrypt the data with my own encryption key.

The "cloud" would give me a chance to restore data in case of catastrophic event causing loss of all hardware (say - flat burned down) but I still have to store the key offsite to be able to do that.

I was thinking of either burning a DVD with the key or printing it in a big font (for OCR or manual entry) and I wonder what would be the best in terms of chance for successfully restoring the key after a few years in storage?

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    A QR code may also be worth thinking about - easier than typing, less chance of error vs OCR. No reason you can't have both text and QR on the same sheet. – Someone Somewhere Jun 13 '16 at 22:04
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    When I first saw this title, I read it as "print and burn", and was wondering why you'd want to set a key on fire. – Bobson Jun 14 '16 at 2:22
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    Get a granite platter and engrave it. That should last long enough and could also withstand a catastrophic flooding or fire. – Alexander Jun 14 '16 at 6:12
  • Could buy a safe which is also fire proof. Storing in a bank safety deposit box is like £18 a month... – k1308517 Jun 14 '16 at 10:46
  • Hopefully you will verify your backup more frequently then "every few years". Part of that means checking on the key. At that time you can replace the copy. – StrongBad Jun 14 '16 at 16:26
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I still can use CDs I've burned down over 10 years ago. There are papers from thousands of years ago as well, so it's really a matter about how good are the conditions where the CD or the paper are stored.

Anyway, why limit yourself to just either of them? Use both.

  • Better yet, lightscribe a QR code on the label side of the DVD – John Dvorak Jun 14 '16 at 5:21
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I always print my encryption keys. The tools I use is first 7-zip to compress and encrypt the key. Then I use PaperBack to print the 7z file. The advantage of this over other tools is, that the scans can be automatically converted back into files without resorting to OCR. This has always worked for me without problems so far. I trust paper printed with a laser printer to last much longer than CDs. Also, with paper a single look allows me to see if it is still intact.

I have a copy of one key at several friends in paper form. With this key, I can decrypt my password save that is stored at several cloud providers. So as long as I remember the password I used to encrypt the 7zip file, I shouldn't lose any data.

I trust my friends to keep that backups relatively save. But if they lose it and if an adversary gets his hands on it, they still would need to break the AES encryption (of 7-zip, I don't trust PaperBack to encrypt properly) for which I used a ~60 letter passphrase.

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Do not rely on a DVD-R only. While they may last a 100 years, they may also only last one if stored improperly or just accidentally scratched when taken out to be used. If you opt for a DVD, store the key on a paper as well.

But the most important question is what you store the DVD or paper in. I would recommend a water tight (protected against flooding) metal (protected against physical damage or hungry mice) box. The next issue is where to store it. The best, but non-free, option is to use a safety deposit box at a bank. If that is to expensive, at least do not store it anywhere near the media the key encrypts.

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    Papers are often stored for decades safely with lawyers or in a safety deposit box where others are liable for the damages in the event they are not available when called upon. Please don't try to protect these expensive documents yourself. – corsiKa Jun 14 '16 at 0:35
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    The biggest mistake people make when trying to protect valuable information is to only make one copy, then rely on guarding that copy against damage. Make a whole bunch of copies. Store one in your desk drawer at work, mail a copy off to your parents, stash a copy in your deposit box at the bank, and so on. – Mark Jun 14 '16 at 2:01
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    @Mark excellent way to leak your key to the world – John Dvorak Jun 14 '16 at 5:22
  • @JanDvorak, a key's no good if you don't know what lock it goes to. – Mark Jun 14 '16 at 20:18
  • @mark are you endorsing security through obscurity? – John Dvorak Jun 14 '16 at 20:26
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Please use proper equipment for these kind of things. In this case a Encryption key.
Store it in a Temper resistant device, like a smart-card or usb-keystore (like this) And make a few 'copies' of the key that you distribute over multiple location.
You secure the keys themselves with a pass phrase you store at a DIFFERENT location as where the USB-keys are.
And you keep one stick behind to make the backups with. in this manner your key is protected from 'random' access (only have the USB in the PC when doing a backup though) Any other means does not protect you from misuse due to either coercion or malice.

  • This way you will soon have to replace your key, because you can't read a key from a smartcard. Assume you have one key and one backup. You accidentally destroy one smartcard. Your key now is on only one smartcard. To be safe, you need to create another smartcard. But your key only is on a smartcard and you can't get it out. Now you need another key and you can only decrypt old files with that one smartcard left. If this also breaks, your data is gone! – Josef Nov 13 '17 at 9:43
  • All HSM have a way to store a copy of the keys in the HSM. as for smart cards, there are ways to make sure you have a copy of the key on the card. It is hardware dependent but doable. As to limit the problems when re-issuing a key. you use the card to encrypt a symmetric passwordfile that you can re-encode to use a diffrent smart-card when key-revocation is used. You could also use newer technology (like hashicorp's Vault) to have it store your secrets and not worry about the details. Invest into the technology involved and learn what works and what doesn't. – LvB Nov 13 '17 at 11:16
  • A HSM costs a lot more than a normal individual is willing to invest. There are easy ways to make sure you have a copy of a key on a smartcard, but then you are back to point 1: Where to store that copy? If you use a nitrokey like you said, there is no way to get the key back out from the key. When the nitrokey is lost/broken, the key on it is gone. – Josef Nov 13 '17 at 13:37
  • Nitrokey has an affordable HSM, and you can put a key on a nitrokey that you than also put on another key / other medium. I suggest you check out the supplier first. shop.nitrokey.com/shop/product/nitrokey-hsm-7 – LvB Nov 13 '17 at 14:22
  • So where do you store the Device Key Encryption Key (DKEK) which is used to restore backups on another nitrokey? – Josef Nov 13 '17 at 14:51

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